In a typical rod rolling mill, as depicted diagrammatically in FIG. 1, billets are reheated in a furnace 10. The heated billets are extracted from the furnace and rolled through a roughing mill 12, an intermediate mill 14, a finishing mill 16 and in some cases, a reducing/sizing mill (not shown). The finished products are then directed to a laying head 18 where they are formed into rings 20. The rings are deposited on a conveyor 22 for transport to a reforming station 24 where they are gathered into coils. While in transit on the conveyor, the rings are subjected to controlled cooling to achieve selected metallurgical properties.
Again, as depicted diagrammatically in FIG. 2, the laying head 18 includes a hollow quill 26 containing a curved laying pipe 28. A bevel gear set 30 powered by a motor 32 serves to rotatably drive the quill and laying pipe about an axis “A”.
Over the last several decades, the delivery speeds of rod rolling mills have increase dramatically. For example, mills now have the capability of rolling 5.5 mm rod at speeds of 110 m/sec. and higher. At such speeds, the hot rolled products exert a punishing effect on the laying pipes, causing internal pipe surfaces to undergo rapid localized frictional wear and premature failure. Also, as the laying pipes wear, their ability to deliver a stable ring pattern to the conveyor 22 deteriorates. Unstable ring patterns disturb cooling uniformity and also contribute to coiling mishaps, commonly referred to as “cobbling”, at the reforming station 24. Frequent and costly mill shutdowns are required to replace prematurely worn laying pipes and to address problems associated with cobbling at the reforming station.
Those skilled in the art have made repeated attempts at increasing the useful life of laying pipes. For example, as disclosed in U.S. Pat. Nos. 4,074,553 and 5,839,684, it has been proposed to line the laying pipes with wear resistant inserts. U.S. Pat. No. 6,098,909 discloses a different approach where the laying pipe is eliminated in favor of a guide path defined by a spiral groove in the outer surface of a conical insert enclosed by a conical outer casing, with the insert being rotatable within the outer casing to gradually shift the wear pattern on the inner surface of the outer casing.
For various reasons, none of these approaches has proven to be a practical solution to the problem of premature pipe wear.
Attempts have also been made at carburizing the interior pipe surfaces in order to increase hardness and resistance to wear. However, the carburizing process requires a drastic quenching from elevated processing temperatures, which can distort the pipe curvature. The carburized layer has also been found to be relatively brittle and to temper down at elevated temperatures resulting from exposure to the hot rolled products.
For a number of years, it has been well accepted that laying pipes with reduced bore sizes provide a number of significant advantages. By radially constricting the hot rolled products within a smaller space, guidance is improved and the ring pattern delivered to the cooling conveyor is more consistent, making it possible to roll at higher speeds. Unfortunately, however, these advantages have been offset to a large extent by significantly accelerated pipe wear. Thus, in the past, those skilled in the art have deemed it necessary to compromise by employing larger bore laying pipes and rolling at reduced speeds below the rated design speeds of the mills.